I'm thinking primarily 1, secondarily 2. Number 3 I don't understand, number 4 sounds counterproductive.
There seems to be conflicting ideas on what they are and how they're used. I first heard the term 'flanking' to describe a maneuver
on old sternwheel towboats. Originally they only had rudders in front of the paddle wheel like this...
When going forward their steering was only using #2, which at times was too weak for manueverability. In the case of pushing a line of barges around turns, (oddly enough, towboats pushed, they didn't tow barges) the flanking manuever developed, which as I understand it amounted to charging into the turn at speed and at the correct time throwing the paddle wheel into reverse and turning the steering wheel in the opposite direction normally required. That would allow the inertia/momentum of the tow to keep it going forward into the turn while the effects of #1 would kick the stern of the tow around until the whole thing was lined up better for the turn.
Eventually they developed 'Monkey Rudders' behind the paddle wheel like this...
which 'aped' the forward rudders and allowed the towboats the effects of #1 without losing forward speed. If you google flanking rudders those aft monkey rudders on sternwheelers are sometimes referred to as flanking rudders. In modern usage with regular propellers, the forward rudders are called the flanking rudders.
I've been told by one person that the two rudder systems turn in opposite directions, but to me that makes no sense at all and would counteract each other. I've seen in google images where for modern rc tugboat models, when going forward only the aft rudders work and when going astern only the forward/flanking rudders work. So they all obviously use #1 primarily, and it seems to me if the opposite rudders also turn in the same direction they would add some benefit from #2 instead of counteracting the other rudders.
I also saw where if the two modern flanking rudders on each side of the propeller were set to 'toe out' from each other a little bit, they tend to funnel water to the prop for a noticeable increase in thrust.